1. Technical Field
The present invention relates to a resin composition suitable for use as a material for manufacturing an optical element, that is, an optical material, and a molded article including the resin composition.
More particularly, the present invention relates to a resin composition suitable for manufacturing an optical element that utilizes its birefringence, and a molded article including the resin composition.
2. Background Art
In recent years, with growth in the display panel market, the demand for a clearer image has increased. To meet this demand, instead of a simple transparent material, an optical material having advanced optical characteristics is required.
One such advanced optical characteristic is birefringence. In general, a polymer has different indices of refraction in a direction of the molecular main chain and in a direction perpendicular to the molecular main chain, and hence birefringence is generated. In certain applications, it is necessary to strictly control this birefringence. For example, to be used as a protective film on a polarizer of a liquid crystal, a polymeric molded article should have a smaller birefringence and evenly transmit all wavelengths of light. A typical example of such a molded article is a film made of triacetyl cellulose. On the other hand, birefringence can be utilized to change linear polarization to circular polarization (a ¼ wavelength plate or the like), or to compensate for the birefringence of the liquid crystal (an optical compensation film such as a retarder film). Polycarbonates are well known for these birefringent uses.
In recent years, liquid crystal displays are getting larger. Accordingly, the polymer optical elements (such as the retarder films) need to get larger. However, in larger displays, greater external pressure is applied to the optical element. Therefore, if the optical element is formed of a material whose birefringence easily changes when external pressure is applied, the birefringence of the optical element will change across the display, resulting in nonuniform contrast.
The extent to which birefringence changes with the application of external pressure is represented by a photoelastic coefficient. The above described polycarbonates have large photoelastic coefficients. Therefore, to replace these polycarbonates, birefringent optical materials having small photoelastic coefficients are desired.
A retarder film made of a styrene-acrylonitrile copolymer is known (Patent Document 1). However, this film has a large photoelastic coefficient, and hence it is not a satisfactory retarder film.
A blend of a styrene-acrylonitrile copolymer and an acrylic resin (Non-Patent Document 1), a blend of a styrene-methacrylic acid copolymer and an acrylic resin (Patent Document 2), and a blend of a styrene-maleic anhydride copolymer and an acrylic resin (Non-Patent Document 2) are known. However, these blends are not intended to be used as optical materials.
A resin composition containing a styrene-maleic anhydride copolymer and a polycarbonate is known (Patent Document 3). However, this resin composition is a combination of resins having positive photoelastic coefficients, and hence an absolute value of the photoelastic coefficient of the composition is large.
A retarder film made of resins having positive and negative inherent birefringences (Patent Document 4) is also known. However, the film which is specifically disclosed in Patent Document 4 is a film formed from a combination of resins having positive photoelastic coefficients, and this kind of film has a large photoelastic coefficient.
Moreover, a retarder film made of resins having positive and negative photoelastic coefficients is also known (Patent Document 5). In this retarder film, however, a resin having a large photoelastic coefficient (60×10−8 cm2/N (=60×10−12 Pa−1) or more) is used as the resin having the positive photoelastic coefficient to provide high birefringence necessary for the retarder film. Therefore, usable materials are limited, and there is a problem that the desired optical characteristics cannot freely be designed.
Furthermore, in recent years there has been a need for controlling not only the in-plane retardation of the retarder film but also controlling the retardation in the thickness direction of the retarder film in order to obtain a higher image quality of a liquid crystal display. Thus, in a retarder film for a horizontal electric field (IPS) mode liquid crystal display which has become more popular in recent years, it is preferable that the retardation in the thickness direction is negative. However, the conventional optical film made of triacetyl cellulose or a polycarbonate, and the retarder film disclosed in Patent Document 5, have retardations in the thickness direction which are positive. Therefore, there is a need for an optical film having a small absolute value of the photoelastic coefficient and a negative retardation in the thickness direction.    Patent Document 1: Japanese Patent Application Laid-Open No. 05-257014;    Patent Document 2: Japanese Patent Application Laid-Open No. 56-98251;    Patent Document 3: Japanese Patent Application Laid-Open No. 7-233296;    Patent Document 4: Japanese Patent Application Laid-Open No. 2002-40258;    Patent Document 5: Japanese Patent Application Laid-Open No. 2004-212971;    Non-Patent Document 1: T. Nishimoto, POLYMER, Vol. 30, p. 1279-1285, 1989; and    Non-Patent Document 2: D. Chopra, Society of Plastics Engineers. Annual Technical Conference, 59th, Vol. 2, p. 2326-2330, 2001.